Create Trusted Remote Desktop Services (RDP) SSL Certificate

For Windows environments that want extra security, one of the features that has been around for ages is requiring TLS 1.0 for Windows RDP (Remote Desktop) connections. This functionality requires a certificate on the server, since TLS is based on the usage of X.509 certificates. Installing a RDP SSL certificate is easy.

By default Windows will create a self-signed certificate automatically for use with RDP. But as we all know, self-signed certificates are nearly worthless, and could easily be intercepted for man-in-the-middle attacks. So one should reconfigure Windows to use a trusted certificate. Thankfully this is fairly easy, and once configured, pushed down to all servers via GPO for automated deployment.

I’ve validated that this procedure works both on Windows Server 2008 R2 and Windows Server 2012. It may work on Windows Server 2008.It requires the use of a Microsoft enterprise online certificate authority. Again, I’ve used both Windows Server 2008 R2 and Windows Server 2012 CAs with success. Not surprising, since certificates are industry standard. For the purposes of this article I’ll use Windows Server 2008 R2 CA, and Windows Server 2012 “target” server.

The general process is first creating a new Certificate Authority certificate template that has an extended key usage to limit its use to only Remote Desktop TLS sessions. Second, we configure a GPO setting to automatically configure servers to request a certificate via this template, and use it for RDP TLS. Refresh GPO on the target server, and finally we attempt to connect via a stand-alone computer to verify it sees the certificate that we deployed.

Installing a RDP SSL Certificate

1. On your Microsoft certificate authority server open the Certificate Templates console.

2. Duplicate the Computer template and use the Windows Server 2003 Enterprise format (Server 2008 v3 templates will NOT work).

3. Change the template display name to RemoteDesktopComputer (no spaces). Verify the Template Name is exactly the same (no spaces). You can use a different name if you want, but both fields must match exactly.

4. Now we need to create an application policy to limit the usage to RDS authentication, then remove the other application uses for the certificate. On the extensions tab click on Application Policies then click on Edit.

5. Click on Add, then click on New.  Set the value of Name to Remote Desktop Authentication. Change the object identifier to 1.3.6.1.4.1.311.54.1.2.

6. From the Application Policies list, select Remote Desktop Authentication.
7. Back on the certificate template properties, remove all other entries. Only Remote Desktop Authentication should be present.
8. If you wish, you can modify the validity period of the certificate, making it say two years instead of the default of one.
9. You probably want to secure your domain controllers as well, so for that we need to modify the security setting on the template. Open the Security tab and add the group Domain Controllers and give the group Read and Enroll (not Autoenroll).
10. Open the MMC snap-in for managing your Certificate Authority and locate the Certificate Templates node. Right click, select New, then Certificate Template to Issue. Choose the RemoteDesktopComputer template.
11. Next up is configuring the GPO to utilize the new template. You can modify any GPO you wish, or create a new one. Obviously the scope of the GPO should cover any servers that you want to secure with TLS. This could be a server baseline GPO, domain GPO, or whatever you want.
12. In the GPO editor locate the node Computer Configuration\Policies\Administrative Templates\Windows Components\Remote Desktop Services\Remote Desktop Session HostSecurity. Modify the Server Authentication Certificate Template setting. Enable the policy and enter the certificate template name that exactly matches what you created in your CA.

13. In the same GPO node, configure the Require use of specific security layer for remote (RDP) connections to use SSL (TLS 1.0).

14. Wait for the GPO to replicate, then refresh the GPO on a test server. Wait a minute, then open the Certificates MMC snap-in for the computer account. Look in the PersonalCertificates store for a certificate that has the Intended Purposes of Remote Desktop Authentication. If it’s not there, wait a minute, and refresh. If it never appears, something is wrong. Look at the gpresult to make sure your GPO is being applied to the server.

15. Once the certificate appears, double click on the certificate to open it. On the Details tab look at the first few characters of the thumbprint value and remember them.

16. To make sure the RDP service is aware of the new certificate, I restart the Remote Desktop Services service.

17. Open an elevated PowerShell prompt and run this command:

Get-WmiObject -class “Win32_TSGeneralSetting” -Namespace root\cimv2\terminalservices -Filter “TerminalName=’RDP-tcp'”

Validate that the Security Layer value is 2 and that the thumbprint matches the certificate. If both of those settings are correct, then you are good to go!

As a quick test I attempted to connect to this server from a non-domain joined computer that did not have the root certificate for my CA. I configured the RDP client to warn on any security issues. As expected, the client threw errors about the CRL not being available, and that it didn’t trust the chain. I also viewed the certificate and verified it was the correct one.

It seems Windows 8 has much more stringent certificate checking than Windows 7. The screenshots below are from Windows 7, in case you didn’t recognize the chrome. When using a Windows 7 non-domain joined computer to access the same TLS protected server, I got NO certificate warnings. That was even with the RDP 8 add-on hotfix. I’m glad to see Win8 does thorough certificate validation.

Connecting to the same server from a domain-joined computer that trusted the root CA resulted in no security warnings and a successful connection. If you look at a Wireshark capture you can also validate that CRL information is being exchanged between the computers, which means TLS is being used.